Brick Tunnel Building Tutorial: Arches, Portals and Lighting

Anyone who builds mountain landscapes eventually hits the same problem: the tunnel. A tunnel that a vehicle can actually drive through, or a figure can walk through, is often the detail that makes an entire MOC land. It is also the single easiest thing to ruin, because a flat beam spanning a square hole reads as fake at a glance. This tutorial covers the full build, from arch geometry and portal proportions through lighting and mountain blending, up to the jump from a single bore to a twin-bore tunnel.

Start With the Arch: Where the Skeleton Comes From

The biggest mistake in brick tunnels is forcing a rectangular opening out of straight beams. Real tunnels are almost always vaulted, and the arch is the first decision you make. The simplest route is curved arch bricks such as 1x4 and 1x6 arch pieces, stacked course by course to form the inner vault. It goes together fast, but the resulting radius is stepped rather than continuous. For a smooth, wide arch, chain 1x2 hinge bricks into a bendable arc band and lock that band to the inner wall bricks with clip plates. Once the inner radius is fixed, wrap the outside in plates and interlock them against each other so the crown is genuinely locked. A crown that is only stacked, not bonded, will shift and collapse under the weight of the mountain above it.

Portal Proportions: What Makes a Tunnel Read as Real

Size the opening so a 4-stud-wide vehicle chassis clears it, and keep the clear height at 6 studs or more. That is the point where the tunnel visually promises that something could walk through it. Beginners routinely shrink the opening by one course, and the whole tunnel instantly looks like a mouse hole. A useful rule of thumb: keep the inner arch radius in roughly a 1:3 relationship with the total tunnel length. For a single-bore tunnel, that means a length of about 16 to 24 studs. Go longer and the passage looks mechanical and dull; go shorter and the tunnel cannot plausibly support the mountain sitting on top of it. Finish the portal with a ring of trim bricks in the same color as the arch, which visually separates the hole from the rock face and immediately adds depth.

Lighting: You Have to Light It From Inside

Rely on exterior light alone and the interior of the tunnel is a permanent black void, which drags down the finished model. Reserve lamp positions inside the wall as you build: leave a deliberate one-stud gap in the inner arch course, tuck an LED strip into it, and cap it with transparent bricks acting as a diffuser. Place two lights, one about 4 studs inside the entrance and one at the midpoint, and stagger their brightness. That contrast is what produces the sense of depth, the feeling that the tunnel keeps going. Run the wiring out through a raised void layer under the tunnel floor. Never route cable across the visible surface of the mountain, or the illusion breaks.

Blending Into the Mountain

The most common failure here is an obvious seam where the tunnel wall meets the hillside. Build mountain first, hole second. Rough out the massing along the contour lines of your mountain, cut the opening where the tunnel module will sit, then fair the slopes around the portal into the surrounding terrain using slope bricks and wedge bricks. At the seam itself, use 1x1 bricks in an offset bond so that courses do not line up vertically from one layer to the next. Lay a few shades of green brick over the surface to suggest vegetation, and the tunnel starts to look grown into the mountain rather than plugged into it.

Going From One Bore to Two

Once a single bore works, the natural upgrade is a twin tunnel. The critical element is the central pier. It needs to be at least 2 studs wide, and its top must be flush with the crowns of both arches so it can carry the large arch that spans both bores. Build one bore first and validate the structure, then mirror it to create the second. Trim the central pier with the same inner arch bricks, and carry the outer frames of both portals into a single continuous eave line; that continuity is what makes the pair read as one structure instead of two tunnels pushed together. If you want a roadway deck on top, span long plates across both arch bands and add vertical supports underneath. Do not leave the deck floating.

Common Failure Points: Check Before You Commit

Run through these before you finalize a build and you will save yourself several teardowns: an arch crown with no interlocking that shifts under load, an opening built one size too small, exposed lighting cable, and seams between mountain and tunnel that run straight up and down. One more that gets overlooked: the floor bricks inside the tunnel need to be the same part and the same height as the road outside, or vehicles will snag and bump at the portal.

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